In the emerging technology known as “Silicon Photonics,” optical devices are integrated with electronic components. A classical “Silicon Photonics” application includes an optical transmitter and an optical receiver and is shown in FIG. 1. A high frequency data stream is transmitted by modulating a laser beam. By way of this beam modulation, an electrical data stream is converted into an optical data stream, which is more suitable for a long-distance low-loss data transmission.
Together with optical modulators, optical switches are commonly integrated with electrical functional circuits in a “Silicon Photonic” integrated circuit. Driving these optical modulators commonly may require both high voltage and high speed capabilities, with the constraint of driving large capacitive loads with very short rise and fall time constants.
An example of these functional circuits is the driver for the Mach Zehnder optoelectronic modulator of FIG. 2. An integrated Mach Zehnder (MZ) optoelectronic modulator is a combination of two directional couplers interconnected by two symmetrical silicon waveguides of a given length. The group velocity of the light into the two waveguides is controlled by the voltage polarization of two varicap diodes. The working principle of the MZ optoelectronic modulator is based on the constructive or destructive interference of the two separated paths as a consequence of the two different light group velocities.
In an MZ optoelectronic modulator, such a selective interference is used to modulate a continuous wave laser beam, but the same principle of constructive or destructive interference is commonly used also to perform other optical functions, like optical switches variable attenuators etc.
The driver is commonly required to have high current capability at high power efficiency. If CL is the value of the capacitance of the MZ optoelectronic modulator, the required capability of the output driver may be approximated as follows:
            I      ⁢                          ⁢      out        =                            C          L                ·        Vout_peak                    t        ⁢                                  ⁢                  r          ⁡                      (                          or              ⁢                                                          ⁢              t              ⁢                                                          ⁢              f                        )                                ,wherein tr and tf are the rise time and the fall time constants, respectively.
FIG. 3 shows a typical application. Due to high speed requirements, high-speed complementary metal-oxide-semiconductor (CMOS) technologies are commonly selected for the electronic part. These may include the draw-back of the voltage capability of the CMOS not being adequate for the voltage levels required for driving the MZ. For this reason, the driver is commonly required to drive the MZ optoelectronic modulator with a voltage swing exceeding the low voltage CMOS supply (Vdd1).
Prior art approaches may include high voltage level shifters to convert the CMOS digital input signals, that are constrained within a low voltage supply (Vdd1), to the output buffer voltage levels (Vout) required to drive the Mach Zehnder optoelectronic modulators, as schematically shown in FIG. 3. Moreover, prior art approaches may also have a fixed output swing without any possibility of adjusting the peak value of the output voltage Vout. U.S. Patent Application Publication No. 2009/0148094 to Kucharski et al. and U.S. Pat. No. 7,450,787 to Kucharski et al. disclose an optoelectronic device having a plurality of optical modulators and a plurality of distributed amplifiers, each electrically coupled to a respective optical modulator. U.S. Pat. No. 7,519,301 to Keil et al. discloses several circuit architectures of emitter follower-based or source follower-based drivers coupled to drive a Mach-Zehnder interferometer optical modulator. U.S. Pat. No. 7,199,617 discloses a level shifting device that can translate an input signal operating in a first voltage range to an output signal operating in other voltage ranges while using transistors rated to withstand the supply voltage.